The present invention relates to a direct illumination apparatus for microscopes for generating illumination for light fields, phase contrasts, interference contrasts and dark fields by means of a deflecting mirror mounted in the path of the observation beam and partly silvered at the center and fully silvered in an annulus, and to an annular condensor associated with the illumination system with an adjustable stop to suppress the light field beam and with optical components for illuminating an objective pupil for the light field and the annular condensor with collimated light for the dark field, a stop acting as a diaphragm in the path of the light field beam and approximately limiting the illumination of the object field in the path of the dark field beam.
The state of the art of incident light illumination instruments for selective light and dark field illumination may be ascertained by reference to U.S. Pat. Nos. 1,943,510 and 3,930,713, and West German Patent Application 2,021,784 of K. P. Schindl, published Nov. 11, 1970, the disclosures of which are incorporated herein.
The novel illuminating apparatus of the present invention is especially suited as an accessory to mass produced microscopes, even to those of simple and compact design.
Direct illumination apparatus of the present invention are disclosed in West German Published Application No. 2,021,784 and U.S. Pat. No. 1,943,510, in which a separate outer dark field beam of annular cross section and an inner light field beam of circular cross section are generated. The illumination beams are deflected into the observation path of the microscope by means of a mirror at 45.degree. to the optical axis of the illumination apparatus. The mirror is fully specular in the region of the incident dark field beam and partly transmitting in that of the light field beam. Because the dark field beam is guided around the light field beam even after it is guided into the observation beam, the cross section of the dark field beam is fairly large.
Mass produced microscopes, especially of small and simple design, are provided with a standardized borehole in the stage which allows passage of the observation beam. The cross section of this borehole is appreciably smaller than the above said cross section of the dark field beam. Therefore the prior art apparatus are unsuited as accessories for these microscopes.
On the other hand, U.S. Pat. No. 3,930,713 discloses a direct illumination instrument in which an annular mirror with reflecting inner surface is mounted after the deflecting mirror to focus the dark field beam within the borehole in the microscope stage. An annular lens mounted in the objective and associated with the illumination instrument guides the dark field beam again around the observation beam. But this instrument assumes that there will be imaging of the aperture stop plane by the dark field beam onto the partly transmitting region of the deflecting mirror. This inevitably causes losses of light and stray light in the dark field beam.